DESCRIPTION: People who inject drugs (PWID) bear a high burden of HIV and hepatitis C virus (HCV) and fare worse along the HIV care continuum. The interplay between individual, network and spatial factors on HIV/HCV risk, particularly in resource-limited settings, has been understudied. Further, most network assessments have been cross-sectional and have focused on egocentric networks. Less is understood about the dynamics of networks over time, relationship of egocentric networks to the broader sociometric or spatial networks, and the impact of these larger networks on disease transmission and health seeking behaviors. India is home to the largest number of opiate users globally; HIV and HCV prevalence is 20% and 50%, respectively. We are currently implementing a cluster-randomized trial to assess the effectiveness of integrated care centers on HIV care continuum outcomes among PWID. In the evaluation of this trial (July - December 2016), we will accrue a sample using respondent driven sampling (RDS) in 12 cities - this will serve as the starting point for this study. The specific ais are: Aim 1: To assess the impact of spatial-, network- and individual-level factors on HIV and HCV incidence among PWID in three Indian cities, selected to reflect diverse HIV/PWID epidemics; Subaim 1a: Use phylogenetic data to characterize HIV/HCV transmission in drug using networks; Aim 2: To assess the social diffusion of behavior change in relation to the HIV care continuum within drug using networks; Aim 3: To develop improved methodology for the application of RDS in PWID populations by overlaying detailed network data on an RDS recruitment process; Subaim 3a: To characterize factors that influence how PWID distribute coupons to members of their networks and the impact of this process on RDS- derived estimates; Subaim 3b: To develop improved RDS recruitment strategies to overcome biases identified in Subaim 3a; Subaim 3c: To determine overlap between the RDS network and phylogenetically-defined clusters to inform the use of RDS for intervention delivery and derivation of population-level estimates; and Aim 4: To establish a platform for future research studies. We will establish a multi-center dynamic cohort across 3 cities with diverse HIV/drug use epidemics in India - Churachandpur, Amritsar and Kanpur. We will characterize the social network of the first 350 participants from the evaluation RDS and recruit those who have injected with these participants in the prior 6 months. We will characterize and recruit eligible network members of new recruits iteratively until no additional new network members are identified (~750 per site; 2250 total). Biometric data, GPS coordinates and network software will facilitate tracking and identifying cross-network linkages. Participants will be followed semi-annually and new network members recruited as identified. Our study is innovative in its ability to track temporal network dynamics, use of novel methodology to characterize sociometric and spatial networks, characterization of network impact on transmission using phylogenetic data, and the social diffusion of health-seeking behaviors related to HIV within PWID networks.